2.1. Tumor Necrosis Factor Alpha
TNF-.alpha., a cytokine produced by monocytes (macrophages) and T lymphocytes, is a key element in the cascade of factors that produce the inflammatory response and has many pleiotropic effects as a major orchestrator of disease states (Beutler, B. and Cerami, A., Ann. Rev. Immunol. (1989) 7:625-655).
The biologic effects of TNF-.alpha. depend on its concentration and site of production: at low concentrations, TNF-.alpha. may produce desirable homeostatic and defense functions, but at high concentrations, systemically or in certain tissues, TNF-.alpha. can synergize with other cytokines, notably interleukin-1 (IL-1) to aggravate many inflammatory responses.
The following activities have been shown to be induced by TNF-.alpha. (together with IL-1); fever, slow-wave sleep, hemodynamic shock, increased production of acute phase proteins, decreased production of albumin, activation of vascular endothelial cells, increased expression of major histocompatibility complex (MHC) molecules, decreased lipoprotein lipase, decreased cytochrome P450, decreased plasma zinc and iron, fibroblast proliferation, increased synovial cell collagenase, increased cyclo-oxygenase activity, activation of T cells and B cells, and induction of secretion of the cytokines, TNF-.alpha. itself, IL-1, IL-6, and IL-8. Indeed, studies have shown that the physiological effects of these cytokines are interrelated (Philip, R. and Epstein, L. B., Nature (1986) 323(6083):86-89; Wallach, D. et al., J. Immunol. (1988) 140(9):2994-2999).
How TNF-.alpha. exerts its effects is not known in detail, but many of the effects are thought to be related to the ability of TNF-.alpha. to stimulate cells to produce prostaglandins and leukotrienes from arachidonic acid of the cell membrane.
TNF-.alpha., as a result of its pleiotropic effects, has been implicated in a variety of pathologic states in many different organs of the body. In blood vessels, TNF-.alpha. promotes hemorrhagic shock, down regulates endothelial cell thrombomodulin and enhances a procoagulant activity. It causes the adhesion of white blood cells and probably of platelets to the walls of blood vessels, and so, may promote processes leading to atherosclerosis, as well as to vasculitis.
TNF-.alpha. activates blood cells and causes the adhesion of neutrophils, eosinophils, monocytes/macrophages and T and B lymphocytes. By inducing IL-6 and IL-8, TNF-.alpha. augments the chemotaxis of inflammatory cells and their penetration into tissues. Thus, TNF-.alpha. has a role in the tissue damage of autoimmune diseases, allergies and graft rejection.
TNF-.alpha. has also been called cachectin because it modulates the metabolic activities of adipocytes and contributes to the wasting and cachexia accompanying cancer, chronic infections, chronic heart failure, and chronic inflammation. TNF-.alpha. may also have a role in anorexia nervosa by inhibiting appetite while enhancing wasting of fatty tissue.
TNF-.alpha. has metabolic effects on skeletal and cardiac muscle. It has also marked effects on the liver: it depresses albumin and cytochrome P450 metabolism and increases production of fibrinogen, 1-acid glycoprotein and other acute phase proteins. It can also cause necrosis of the bowel.
In the central nervous system, TNF-.alpha. crosses the blood-brain barrier and induces fever, increased sleep and anorexia. Increased TNF-.alpha. concentration is associated with multiple sclerosis. It further causes adrenal hemorrhage and affects production of steroid hormones, enhances collagenase and PGE-2 in the skin, and causes the breakdown of bone and cartilage by activating osteoclasts.
In short, TNF-.alpha. is involved in the pathogenesis of many undesirable inflammatory conditions in autoimmune diseases, graft rejection, vasculitis and atherosclerosis. It may have roles in heart failure and in the response to cancer. For these reasons, ways have been sought to regulate the production, secretion, or availability of active forms of TNF-.alpha. as a means to control a variety of diseases.
The prime function of the immune system is to protect the individual against infection by foreign invaders such as microorganisms. It may, however, also attack the individual's own tissues leading to pathologic states known as autoimmune diseases. The aggressive reactions of an individual's immune system against tissues from other individuals are the reasons behind the unwanted rejections of transplanted organs. Hyper-reactivity of the system against foreign substances causes allergy giving symptoms like asthma, rhinitis and eczema.
The cells mastering these reactions are the lymphocytes, primarily the activated T lymphocytes, and the pathologic inflammatory response they direct depends on their ability to traffic through blood vessel walls to and from their target tissue. Thus, reducing the ability of lymphocytes to adhere to and penetrate through the walls of blood vessels may prevent autoimmune attack, graft rejection and allergy. This would represent a new therapeutic principle likely to result in better efficacy and reduced adverse reactions compared to the therapies used today.
Atherosclerosis and vasculitis are chronic and acute examples of pathological vessel inflammation. Atherosclerosis involves thickening and rigidity of the intima of the arteries leading to coronary diseases, myocardial infarction, cerebral infarction and peripheral vascular diseases, and represents a major cause of morbidity and mortality in the Western world. Pathologically, atherosclerosis develops slowly and chronically as a lesion caused by fatty and calcareous deposits. The proliferation of fibrous tissues leads ultimately to an acute condition producing sudden occlusion of the lumen of the blood vessel.
TNF-.alpha. has been shown to facilitate and augment human immunodeficiency virus (HIV) replication in vitro (Matsuyama, T. et al., J. Virol. (1989) 63(6):2504-2509; Michihiko, S. et al., Lancet (1989) 1(8648):1206-1207) and to stimulate HIV-1 gene expression, thus, probably triggering the development of clinical AIDS in individuals latently infected with HIV-1 (Okamoto, T. et al., AIDS Res. Hum. Retroviruses (1989) 5(2):131-138).
Hence, TNF-.alpha., like the inflammatory response of which it is a part, is a mixed blessing. Perhaps in understanding its physiologic function, one may better understand the purpose of inflammation as a whole and gain insight into the circumstances under which "TNF-.alpha. deficiency" and "TNF-.alpha. excess" obtain. How best to design a rational and specific therapeutic approach to diseases that involve the production of this hormone may thus be closer at hand.